This invention relates to an echo canceller or cancelling circuit for use in a long-distance telephone network and, more specifically, in a long-distance conference communication system.
In a long-distance telephone network, an echo canceller is coupled to a combination of transmitter and receiver. The echo canceller receives a receive-in signal from a remote party through a receiving path, feeds a receive-out signal to the receiver, receives a send-in signal from the transmitter, and delivers a send-out signal to the remote party through a sending path.
The send-in signal comprises an audio signal when speech or conversation is input to the transmitter. The audio signal becomes the send-out signal. The receive-out signal corresponds to the receive-in signal. The receive-in signal also comprises an audio signal which is produced by the remote party and causes the receiver reproduce audible sound. The audio signal comprised by the send-in and the receive-in signals, will herein be called an acoustic and a voice signal, respectively, merely for convenience of description.
A part of the receive-in signal tends to leak into the send-out signal as an echo signal in the manner known in the art. The echo canceller is for cancelling the echo signal.
Long-distance conference communication systems have become known in the art. Such systems are effective for holding a conference or meeting of participants or attendants living in remote locations because the participants need not be present in a single conference or meeting. The system is for use in connecting a plurality of auditoriums by a wired, a radio, and/or a satellite communication network.
In a long-distance conference communication system, a loudspeaker is used as the receiver. At least one microphone is used as the transmitter. The loudspeaker is unavoidably acoustically coupled to the microphone. It is inevitable that a part of the audible sound reproduced by the loudspeaker is picked up by the microphone resulting in a reverberation signal in the send-in signal. The reverberation signal is transferred to the send-out signal and sent back to the remote party. In an unfavorable case, the reverberation signal gives rise to howling. At any rate, the reverberation signal appears from the receive-in signal as an echo signal of a sort in the send-out signal.
For use in a long-distance conference communication system, an echo canceller should therefore exempt the send-out signal from the echo signal which comprises the reverberation signal. It should be noted in this connection that the echo signal resulting from the reverberation signal may have a considerably long duration.
An echo cancelling circuit is disclosed in U.S. patent application Ser. No. 393,893 filed June 30, 1982, by Takashi Araseki et al, assignors to the present assignee. As will later be described with reference to one of a few figures of the accompanying drawings, the echo cancelling circuit is for exempting the send-out signal substantially from the echo signal which may or may not comprise a reverberation signal.
As discussed in the above-referenced patent application, it is possible to understand that the receive-in, the reverberation, and the send-in signals share a lower and a higher frequency band in common. The lower and the higher frequency bands are lower and higher than a predetermined frequency as, for example, 1.7 kHz.
According to the above-cited patent application, attention is directed to the fact that each of the receive-in, the reverberation, and the send-in signals has a considerably higher intensity or power spectrum in the lower frequency band than in the higher frequency band. In view of this fact, the echo cancelling circuit comprises a filter circuit responsive to the reverberation signal for separately producing a lower and a higher reverberation component in the lower and the higher frequency bands, another filter circuit responsive to the receive-in signal for separately producing a lower and a higher receive-in component in the lower and the higher frequency bands, a self-adaptive echo canceller responsive to the lower reverberation component and the lower receive-in component for self-adaptively cancelling the lower reverberation component to produce a lower send-out component, echo suppressing means responsive to the higher reverberation component and the higher receive-in component for suppressing the higher reverberation component to produce a higher send-out component, and send-out signal producing means responsive to the lower and the higher send-out components for producing the send-out signal.
The self-adaptive echo canceller is of a digital type. Other components of the prior-art echo cancelling circuit are analog circuits. As a consequence, the prior-art echo cancelling circuit is still bulky and is not completely reliable. Moreover, the echo suppressing means is made to respond to the higher reverberation and receive-in components. This means that detection must be carried out in the higher frequency band as regards double talk, namely, as regards the presence and absence of the above-described acoustic or voice signal. Inasmuch as the intensity is weak in the higher frequency band, this operation is liable to misdetection.